Rapid and safe start-up from sub-zero temperatures for polymer electrolyte membrane fuel cells (PEMFCs) is a crucial know-how to guarantee these fuel cells' operation under severe weather conditions and to prolong their lifetime. Taking into consideration the disparity in thermal properties, different rising time in temperature and responses to preheating method and control strategy between the membrane electrode assembly and stack hardware, a phase-in preheating method is proposed in the present study. A series of cold start-up experiments for a 2 kW stack were carried out with different preheating methods, including air preheating, coolant preheating, air and end plate preheating under -10 degrees C and -20 degrees C temperature conditions. The experimental results verified the phase-in preheating strategy, and conclude that the corresponding optimal preheating method is to first preheat the stack by air and the end plates simultaneously, and to switch on the stack operation at suitable timing when the stack's temperature is above -5 degrees C. It is also found that when the stack temperature reaches a threshold value, 24 degrees C in the present study, humidification of the reactants and circulation of coolant can be started to bring the stack to normal operating conditions. A thermal analysis for energy balance in the system was performed to gain insight to the problem. By comparing the heat transfer by different fluid (air and coolant) and parts (end plates), it is found that preheating by air along is slow, whereas heating the end plates is fast and time-efficient. Furthermore, it becomes clear that the optimal preheating method is to utilize the waste heat efficiently, which can be achieved by switching on the stack operation at proper timing. (C) 2018 Elsevier Ltd. All rights reserved.